Flow sensor with self heating sensor elements

Abstract

Traditional flow sensors include an upstream resistive sensor element, a downstream resistive sensor element and an intervening heater resistive element. To help reduce the size and/or cost of such flow sensor, it is contemplated that the heater resistor may be eliminated. When so provided, the space required for the heater resistive element, as well as the corresponding heater control circuit, may be eliminated. This can reduce the cost, size and complexity of the flow sensor.

1 .- 20 . (canceled) 
     
     
         21 . A method for operating a flow sensor comprising:
 supplying a current to a bridge circuit comprising a first upstream resistive element connected in parallel to a first downstream resistive element, wherein the current causes the first upstream resistive element to be heated above an ambient temperature, wherein the current does not cause the first downstream resistive element to be heated above the ambient temperature; and   detecting a differential output from the bridge circuit.   
     
     
         22 . The method of  claim 21 , wherein the bridge circuit further comprises a second upstream resistive element connected in series to the first upstream resistive element, wherein the current causes the second upstream resistive element to be heated above the ambient temperature. 
     
     
         23 . The method of  claim 21 , wherein the bridge circuit further comprises a second upstream resistive element connected in series to the first upstream resistive element, wherein the current does not cause the second upstream resistive element to be heated above the ambient temperature. 
     
     
         24 . The method of  claim 21 , wherein the bridge circuit further comprises a second downstream resistive element connected in series to the first downstream resistive element, wherein the current causes the second downstream resistive element to be heated above the ambient temperature. 
     
     
         25 . The method of  claim 21 , wherein the bridge circuit further comprises a second downstream resistive element connected in series to the first downstream resistive element, wherein the current does not cause the second downstream resistive element to be heated above the ambient temperature. 
     
     
         26 . The method of  claim 21 , wherein the first upstream resistive element is associated with a first resistance that changes with temperature, wherein the first downstream resistive element is associated with a second resistance that changes with temperature, wherein a temperature difference between the first upstream resistive element and the first downstream resistive element causes an imbalance in the bridge circuit that corresponds to a fluid flow rate of a fluid. 
     
     
         27 . The method of  claim 21 , wherein a first resistance value of the first upstream resistive element is 500 ohms, wherein a second resistance value of the first upstream resistive element is 500 ohms. 
     
     
         28 . The method of  claim 27 , wherein the differential output is between 96 megavolts and 134 megavolts. 
     
     
         29 . The method of  claim 28 , wherein the differential output is in response to a bridge voltage of 2.4 volts. 
     
     
         30 . The method of  claim 21 , wherein the first upstream resistive element is positioned in a first parallel arrangement with a slit, adjacent a first side of the slit, and without intervening heater element, wherein the first downstream resistive element is positioned in a second parallel arrangement with the slit, adjacent a second side of the slit, and without intervening heater element. 
     
     
         31 . A flow sensor comprising:
 a bridge circuit comprising a first upstream resistive element connected in parallel to a first downstream resistive element,   wherein the bridge circuit is configured to supply a current to each of the first upstream resistive element and the first downstream resistive element,   wherein the current causes the first upstream resistive element to be heated above an ambient temperature, and   wherein the current does not cause the first downstream resistive element to be heated above the ambient temperature.   
     
     
         32 . The flow sensor of  claim 31  further comprising:
 a second upstream resistive element connected in series to the first upstream resistive element, wherein the current causes the second upstream resistive element to be heated above the ambient temperature. 
 
     
     
         33 . The flow sensor of  claim 31  further comprising:
 a second upstream resistive element connected in series to the first upstream resistive element, wherein the current does not cause the second upstream resistive element to be heated above the ambient temperature. 
 
     
     
         34 . The flow sensor of  claim 31  further comprising:
 a second downstream resistive element connected in series to the first downstream resistive element, wherein the current causes the second downstream resistive element to be heated above the ambient temperature. 
 
     
     
         35 . The flow sensor of  claim 31  further comprising:
 a second downstream resistive element connected in series to the first downstream resistive element, wherein the current does not cause the second downstream resistive element to be heated above the ambient temperature. 
 
     
     
         36 . The flow sensor of  claim 31 , wherein the first upstream resistive element is associated with a first resistance that changes with temperature, wherein the first downstream resistive element is associated with a second resistance that changes with temperature, wherein a temperature difference between the first upstream resistive element and the first downstream resistive element causes an imbalance in the bridge circuit that corresponds to a fluid flow rate of a fluid. 
     
     
         37 . The flow sensor of  claim 31 , wherein a first resistance value of the first upstream resistive element is 500 ohms, wherein a second resistance value of the first upstream resistive element is 500 ohms. 
     
     
         38 . The flow sensor of  claim 37 , wherein the bridge circuit is configured to produce a differential output between 96 megavolts and 134 megavolts. 
     
     
         39 . The flow sensor of  claim 38 , wherein the bridge circuit is configured to produce the differential output in response to a bridge voltage of 2.4 volts. 
     
     
         40 . The flow sensor of  claim 31  further comprising:
 a slit having a first side and a second side opposite to the first side, wherein the first upstream resistive element is positioned in a first parallel arrangement with the slit adjacent the first side without intervening heater element, wherein the first downstream resistive element is positioned in a second parallel arrangement with the slit adjacent the second side without intervening heater element.